Sunday, September 27, 2015

Two big events in the history of the Feather River were the discovery of gold at Bidwell Bar in 1848 and the construction of the Feather River Railroad through the Sierras from 1906 to 1909. The railroad opened up the river for a series of dams starting with the Big Bend Dam in 1908 and culminating in the Oroville Dam in 1968. The building of the Oroville Dam required the Western Pacific Railroad to build a new alignment (and new bridges) from Oroville to Big Bend. The new bridges included the Thermalito Diversion Pool Crossing, the lower deck of the West Branch Bridge that we studied last month, and today's North Branch Bridge.

Railroad bridges can often be purely utilitarian structures but the North Branch Bridge is a happy exception. It's a reinforced concrete bridge composed of three parabolic arch spans supporting wide spandrel pier walls and a concrete deck. At 990 ft long it is one of the longest reinforced concrete railroad bridges in the United States. I wonder what possessed the Western Pacific Railroad to build such a beautiful bridge in the middle of the wilderness?

It seems like we feel more affinity for bridges that were built for a certain mode of travel. The Happy Pontist likes pedestrian bridges because they afford designers more freedom of expression and (perhaps) because people traveling on foot or bicycle seem more positive and friendly. We have less interest in highway bridges because of their cookie cutter appearance and because they carry vehicles with fewer positive connotations. The most passionate group are people who love railroad bridges and the trains they carry. People often line railroad tracks to photograph trains crossing over bridges, especially if they are being pulled by an iconic locomotive (photo by Keith Ardinger shown above). However, only freight trains currently travel along the Feather River Canyon. On rare ocassions 'fan' trains with vintage cars and steam locomotives will cross over the North Fork Bridge (photo shown below by Dave Stanley).

Sunday, September 20, 2015

Just downstream of the Oroville Dam are the Upper Thermalito/Table Mountain Bridges. The truss bridge was built in 1907 and carried Business State Route 70 until it was replaced by the adjacent box girder bridge in 1982 (the older bridge was retained for pedestrian and bicycle use).

The history of bridges across this part of the Feather River is a history of floods. The first bridge at this site was a covered bridge that was built in 1870 (see photo above courtesy of CDWR). Truss spans were eventually added to the bridge, but it was destroyed by flooding in 1907. It was replaced by the current truss bridge, which was inundated (but not damaged) by flooding in October of 1962, just before the dam was built.

The 1907 bridge is a 665 ft long, three span Parker (camelback) through truss bridge, with a 215 ft long river span. The adjacent Feather River Bridge (12C0221) is a five span prestressed concrete continuous box girder bridge with a slightly longer (218 ft long) center span. I like how both bridges have the same hexagonal-shaped concrete piers. In the photo above we can see Table Mountain in the distance. Thermalito is a town just west of Oroville. There's a fish hatchery and a smaller dam just below Table Mountain in the photo.

Sunday, September 13, 2015

The two Pulga Bridges are far enough upstream across the North Fork of the Feather River to not have been impacted by the construction of the Oroville Dam. The taller bridge is a truss arch that was built in 1932 and got some repairs and a seismic retrofit in 2002. It carries State Route 70 over the river and over the Western Pacific (now Union Pacific) Railroad Bridge 170 ft below. This portion of State Route 70 was established as the 'Feather River Scenic Byway' by the USDA Forest Service and includes about 130 miles of road carved out of the Feather River Gorge through the Sierra Nevada.

The photo above was taken in 1937 just after the new highway was completed (courtesy of the UC Davis 'Eastman's Originals Collection') . It looks very similar to the photo I took 78 years later. The bridges were named after the nearby town of Pulga (Spanish for flea) which was also called Big Bar.

The railway crossing is a 435 ft long three span through truss bridge on concrete piers. The bridge was built at this location so the grade could be maintained at 1.0 percent all the way to Beckwourth Pass near Nevada. The railway line was completed in 1909 and it's only half as steep as Southern Pacific's Donner Pass Line to the south.

The highway crossing (12 0038) has a 350 ft long main span and a 684 ft long total length. The approach spans are steel girders on steel towers and the main span is a truss arch on concrete piers. Large 'deadmen' supported the ends of the arch while it was assembled. It took 3 1/2 years to build. The bridge deck is on reverse horizontal curves and it also has a twisting superelevation (see photo above).

My friend Steve designed the retrofit/repairs. He said there weren't any shop drawings so he had to measure everything on the bridge (especially all the gusset plates). They built a timber walkway on one side of the bridge so they could work on the retrofit/repairs. Rope climbing teams used to do the biannual bridge inspection but now they use a snooper truck instead (see photo above).

Sunday, September 6, 2015

If the scenery is nice, the bridge's appearance becomes less important. Tall piers and a long straight superstructure suffice to make the 50 year old Canyon Creek Bridge look good crossing this rugged landscape.

The Canyon Creek Bridge (12 0184) is a 780 ft long, four span, steel girder bridge, on tall concrete piers. The bridge has a narrow (28 ft wide) deck without shoulders. The barrier includes a narrow sidewalk that pedestrians can use to avoid traffic (1500 vehicles a day). A short radius curve and considerable terracing was required to squeeze State Route 162 along the east side of the creek.